The present invention relates to the application of tungsten carbide coatings to photo-engraved roto-gravure cylinders, especially roto-gravure printing cylinders.
Roto-gravure cylinders are commonly employed as metering or anilox rolls to transfer uniform coatings of ink or the like, and as printing rolls to transfer a specific ink pattern. Such cylinders are generally fabricated by a photo-engraving process wherein a cellular surface is produced on the cylinder periphery which functions to receive ink from a source, and transfer it to a web or another roll.
Since roto-gravure rolls are subject to rapid wear, and to the corrosive effects of printing inks, it has been common to plate such rolls with substances such as copper or chrome to maximize durability. Plating with copper and chrome is very expensive and does not provide as much durability as a tungsten carbide coating. Tungsten carbide coatings have been applied with some success to mechanically engraved metering rolls by a flame spray technique, but such success has not been achieved in connection with roto-gravure printing rolls. In that regard, the presently utilized flame spray techniques result in the application of a coating which is excessively thick for roto-gravure printing rolls and thus diminishes the definition of the cell pattern.
For example, the disclosure of a tungsten carbide coating of 0.002 to 0.008 inches on a metering roll in U.S. Pat. No. 4,009,658 would be excessively thick for a gravure printing roll. Although the cells are not filled to such an extent as to unduly impair the performance of a gravure metering roll, a gravure printing roll coated to this thickness cannot perform at acceptably high levels of quality in most instances. Accordingly, gravure printing rolls continue to be coated with more costly and less durable copper and substances, such as chrome, which can be applied in sufficiently thin coatings.
When applying coatings of substances such as chrome to copper roto-gravure printing cylinders, grit-blasting step has been performed to form a pitted surface on the copper surface to facilitate adherence thereto of the coating, the latter entering the surface pits to create a mechanical bond between the coating and the cylinder. Prior to the application of the coating, the copper surface is thoroughly cleaned and special precautions are taken to prevent oxidation of the copper surface since the formation of a copper oxide film has been herefore considered to be detrimental. It would be desirable to eliminate the need for the grit-blasting step which adds appreciably to the fabrication costs of the cylinder.
It is therefore, an object of the present investigation to provide novel methods and apparatus for coating roto-gravure cylinders.
It is another object of the invention to enable tungsten carbide to be applied to roto-gravure printing cylinders, especially roto-gravure printing cylinders, in a relatively thin layer.
It is a further object of the invention to enable a tungsten carbide coating to be flame sprayed onto a roto-gravure cylinder at a thickness in the range of from 15 to 35 microns.
It is an additional object of the present invention to eliminate the need for a grit-blasting step for the application of wear-resistant coatings to copper roto-gravure cylinders and rolls.
It is yet another object of the invention to create a chemical bonding between coatings, such as of tungsten carbide, and copper roto-gravure cylinders and rolls.